A steerable drilling system is used to control the direction a borehole is drilled. Steerable drilling systems include both bent housing systems and rotary steerable systems. Bent housing systems, in particular, conventionally utilize a bent housing in combination with a downhole motor (i.e. a “mud motor”). The bent housing may include a fixed bend or an adjustable bend. Adjusting an angle of the bend on a bent housing conventionally involves tripping out of the well. The mud motor may be selectively powered by drilling fluid pumped from the surface to rotate the drill bit.
To drill a straight section of the borehole with a bent housing system, the drill string is rotated from the surface, without operating the mud motor, so that the bent housing rotates along with the bit about an axis of bit rotation. To change the direction the borehole is drilled, rotation of the drill string is ceased, with the bent motor at a selected rotational position. With the bent motor at the selected rotational position, the bit is then rotated using only the mud motor, to form the deviated section at an angle to the previously-drilled straight section, as guided by the bent housing. The deviated section is drilled until a desired direction is achieved. Once the desired direction is achieved, rotation of the bit using the mud motor is ceased and rotation of the drill string from the surface is resumed to drill another straight section.